Geant4 fast and full simulation for Future Circular Collider studies

Software for the next generation of experiments at the Future Circular Collider (FCC), should by design efficiently exploit the available computing resources and therefore support of parallel execution is a particular requirement. The simulation package of the FCC Common Software Framework (FCCSW) makes use of the Gaudi parallel data processing framework and external packages commonly used in HEP simulation, including the Geant4 simulation toolkit and the DD4HEP geometry toolkit. Using Geant4 for full simulation implies taking into account all physics processes for transporting the particles through detector material and this is highly CPU-intensive. At the early stage of detector design and for some physics studies such accuracy is not needed, making a fast parametrised simulation better suited. Geant4 provides the tools to define a parametrisation, where the overall response of the detector is simulated in a parametric way. Many experiments create their own frameworks for these fast simulation studies. The implementation for the FCC allows an interplay between the two types of simulation within Geant4. Based on the type of the particle or a region within the detector, either full or fast simulation may be performed, running the time-consuming detailed simulation only for the regions and particles of interest.